Question

Suppose I have a model Stock that has several StockPartition (its a property called partitions, its an Array).

The Stock model has a property usedAmount that should change when any of all partition.amount changes, and, of course, be updated when a partition is added/removed.

Example :

stock.get('usedAmount') -> 0
stock.get('partitions') -> [Class, Class, Class]
stock.get('partitions')[0].set('amount', 12)
stock.get('usedAmount') -> I want here to return 12
stock.get('partitions')[1].set('amount', 12)
stock.get('usedAmount') -> I want here 24

How could Stock observes each partitions.amount?
I can write a function addPartition that looks like this :

addPartition: function(partition) {
  partition.addObserver('amount', function() {
    this.get('owner').notifyPropertyChange('usedAmount');
  });
}

But I hope there is a better solution.

Answer 1

I would make usage of the powerful computed properties. Also you can make usage of the helpful methods available on Ember.Enumerable‘s, see http://jsfiddle.net/pangratz666/BxyY4/:

App.partitionsController = Ember.ArrayProxy.create({
    content: [],

    addPartition: function(amount) {
        this.pushObject(Ember.Object.create({
            amount: amount
        }));
    },

    usedAmount: function() {
        // reduce by adding all 'amount' values, start with initial value 0
        return this.reduce(function(previousValue, item) {
            return previousValue + item.get('amount');
        }, 0);
    }.property('@each.amount')
});

The reduce is documented in the docs.